1. Field of the Invention
The present invention relates to lasers. More specifically, the present invention relates to eye-safe lasers.
2. Description of the Related Art
Solid-state lasers often employ a doped-insulator lasing medium. Both glass and crystal mediums have been used. The input power source to the lasing medium is pumplight energy, which is optically coupled into the doped medium. The pumplight energy may be derived from high power light emitting diode arrays, flash lamps or other sources. The pumplight energy raises the energy level of dopant ions within the lasing medium.
Lasing mediums used in accordance with the prior art often include dopant ions dispersed within the host glass or crystal. For example, ytterbium doped yttrium aluminum garnet (Yb:YAG), neodymium doped yttrium aluminum garnet (Nd:YAG) or erbium doped yttrium aluminum garnet or glass (Er:YAG, Er:Crystal Er:Glass). Solid state lasers have been devised that are comprised of a single oscillator/output stage or an oscillator stage followed by one or more amplifier stages.
In each amplifier stage of a laser system, a laser beam from an oscillator or previous amplifier stage is directed into an entrance end of a gain medium. The driving laser beam is directed through the medium and a lasing action occurs when the dopant ions release energy to the beam as they revert to their previous stable low-energy state. A portion of the energy released is converted into light and results in an energy gain in the laser beam traversing the slab. In single stage oscillator lasers, the output beam is taken directly from the oscillator stage.
Solid state lasers have been designed to operate at various wavelengths, with the infrared bands proving to be particularly useful. At any given wavelength, there is some level of fluence, or energy, that represents a threshold of damage to the human retina. However, the band of wavelengths from about 1.4 microns to 1.8 microns has been shown to require energy levels that are several orders of magnitudes greater before the threshold of eye-damage is reached. In fact, this band has been deemed the “eye-safe” band by certain US government agencies. Thus, in operational environments where humans are present, eye-safe lasers are preferred because significantly higher laser beam fluences are permissible.
It is naturally desirable that lasers be designed for maximum efficiency while still meeting target cost and reliability constraints. Two important aspects of system efficiency are the electrical-to-optical efficiency of the pumplights and the conversion efficiency of the lasing medium. An important conversion efficiency measure in diode pumped solid-state lasers is the “quantum defect” of the system. Quantum efficiency is essentially controlled by the wavelength difference between the pump light energy and the laser beam produced. Basically, quantum defect is determined by subtracting the wavelength of the emitter light from the wavelength of the pump light, normalized within the wavelength of the pump light. This yields the so-called quantum defect, as a percentage, which is the theoretically highest efficiency realizable from the systems.
Clearly then, a system in which the pump light wavelength is close to the laser wavelength is preferred from an efficiency perspective. For example, in the case of a one micron Nd:YAG laser pumped by 800 nanometer pump light diodes, a quantum efficiency of less than 80% is realized. In another example in the prior art, a resonantly pumped laser, such as a Yb:YAG laser, which employs 940 nanometer diodes and lases at 1 micron, will yield a quantum efficiency that is greater than 90%. Therefore, everything else being equal, it is preferable that the pumplight wavelength be as close to the lasing wavelength as practicable. There are other factors supporting this desirability as well. For example, ultimately, the difference in the energy between the pumplight and the laser determines the heat loading of the lasing medium, which limits maximum beam output power.
With respect to eye-safe lasers designed to emit in the 1.4 to 1.8 micron band, it is preferable to employ pump light diodes near the laser wavelength However, implementation of an efficient direct resonantly pumped laser, such as an Er:Crystal laser, has been limited by the lack of affordable high power 1500 nanometer diode arrays. Conversely, efficient diode pumped Nd:YAG lasers, as well as Yb:YAG lasers are readily available with commercial off-the-shelf high power 800 nanometer or 940 nanometer diode sources. Thus, the economics of available solid state pumplight sources in the eye-safe wavelength range has caused designers of prior art eye-safe laser to seek an alternate path in system design.
The basic approach to prior art eye-safe laser design has been to employ an efficient resonantly pumped 1 micron laser and a non-linear device that shifts, or converts, the laser beam wavelength into the eye-safe region of the spectrum. Two available non-linear conversion devices are the RAMAN cell and optical parametric oscillators (“OPO”). Thus, prior art eye-safe lasers employ indirect conversion as opposed to direct conversion. Direct conversion means there is a laser medium, such as Nd:YAG or Yb:YAG, in which the pumplight energy is directly converted to the laser beam wavelength within the lasing medium through the lasing action. An Nd:YAG laser employs a laser transition that lases at 1.064 micron with 840-900 nanometer pumplight source. Thus, when the laser medium is excited, it lases directly at 1.064 micron. However, in the case of the eye-safe band, it is not possible to get the Nd:YAG medium lase directly at 1.5 micron. Therefore, the aforementioned non-linear conversion medium is added to the system. In prior art eye-safe lasers, the non-linear conversion medium converts the 1 micron radiation to 1.5 micron. The non-linear conversion process can be accomplished in various mediums, including solid crystal, liquid, or gaseous mediums. For example, methane can be used to produce the 1.5 micron wavelength.
Because of the aforementioned design and cost trade-off, one current state-of-the-art eye-safe laser design is based on OPO-shifted Nd:YAG lasers configured as a mono-block solid-state structure, which is taught in U.S. Pat. No. 6,373,865 dated Apr. 16, 2002 to Nettleton for PSEUDO-MONOLITHIC LASER WITH INTRACAVITY OPTICAL PARAMETRIC RESONATOR, the teachings of which are hereby incorporated by reference. This design generally has good efficiency, however, it is inherently bulky and cumbersome because it requires several high-power diode bars to enable it to operate with an appreciable energy output. Furthermore, OPO based energy conversion is inherently inefficient and suffers from compromised beam quality. Direct eye-safe lasers based on erbium dopants are also known in the art. Erbium (Er) lasers utilize a ytterbium-erbium energy transfer pumping mechanism in a phosphate-glass host. However, the glass host is severely power-limited by its poor thermal properties such that operating these lasers at higher average powers is prohibited. Attempts to reproduce this functional ytterbium-erbium energy transfer pumping process in a crystal host (such as YAG) has resulted in a severely limited laser performance. This limitation is articulated in the reference by T. Schweizer, T. Jensen, E. Heumann, and G. Huber, “Spectroscopic properties and diode pumped 1.6 μm performance in Yb-codoped Er:Y3Al5O12 and Er:Y2SiO5”, Optics Communications, 118, 557-561 (1995). This is due to the fact that the energy level dynamics of erbium in a crystal host is much less favorable than that in phosphate glass.
Thus, there is a need in the art for a system and method to produce an eye-safe laser that offers the performance characteristics of the Nd:YAG laser while still efficiently producing an eye-safe beam utilizing an intra-cavity energy conversion that is inherently compact and low-cost allowing for significant scaling of the output energy and power with greatly reduced numbers of diode pumplight sources.